In an outboard engine, generally speaking, the exhaust gases are discharged through an exhaust pipe into an exhaust expansion chamber in a casing until they are discharged into water from the lower portion of the casing. In order to cool that portion of the wall surface of the exhaust expansion chamber and other structure which is to be exposed to the exhaust gases, water which was used to cool the engine is discharged into the exhaust expansion chamber, or some water before the engine cooling operation is discharged into the expansion chamber from the vicinity of a water pump for pumping the cooling water until it is discharged into the water together with the aforementioned exhaust gases.
In an outboard engine thus constructed, the cooling water discharged into the exhaust expansion chamber is expanded in the form of droplets into the expansion chamber or wets the outer circumferential wall of the exhaust pipe. In this exhaust pipe, on the other hand, the high pressure waves, which are generated at the instant when the exhaust port of the engine is opened, are propagated in the exhaust pipe toward the leading end thereof, and those positive pressure waves are changed, at the instant when they expand from the leading end of the exhaust pipe into the expansion chamber, into negative pressure waves, which in turn return toward the exhaust port through the exhaust pipe, thus establishing the pulsating phenomena of the positive and negative pressure waves.
As a result, when a vacuum is established in the exhaust pipe, the aforementioned water droplets floating in the vicinity of the leading end of the exhaust pipe and/or the water droplets wetting the outer circumferential wall of the exhaust pipe and reaching the opened lower end of said pipe along the wall surface may be sucked into the exhaust pipe and they may steal into the cylinder of the engine, thus causing a misfire. Especially in case the outboard engine is used on the sea, the brine may wet the inner wall of the exhaust pipe to invite difficulties such as corrosion or increase in passage resistance.